What role does a trial judge have in instructing a jury in a criminal trial?

The trial judge's instructions to the jury have always been recognized to be a fundamentally important stage of the criminal proceeding. (See, e.g., People v. Dagnino (1978) 80 Cal.App.3d 981, 988, 146 Cal.Rptr. 129.) Indeed, one can legitimately argue that the primary function of the judge in a jury trial is to explain the applicable legal principles in such a way as to focus and define the factual issues which the jury must resolve. In this role, the trial judge acts much like a teacher or a guide; it is no accident that we refer to the trial court's obligation to "instruct" the jury on the applicable law. It is not sufficient that the trial judge be an adequate legal lecturer. Jurors are not first year law students with some independent motive for legal study. At best, they are well-meaning but temporary visitors in a foreign country attempting to comprehend a foreign language.